The invention relates to transport temperature control units, and more particularly to a method of controlling the compressor of a transport temperature control unit.
Straight trucks and tractor-trailers (hereinafter referred to as xe2x80x9cvehiclesxe2x80x9d) frequently transport cargo that must be maintained within a predetermined temperature range during transportation in order to preserve the quality of the cargo. Vehicles that transport such temperature-sensitive cargo have a conditioned space that is maintained within the predetermined temperature range by a temperature control unit. The operation of the temperature control unit is monitored and controlled by an electronic controller.
One component of the temperature control unit is a compressor that raises the pressure of the refrigerant circulating through the temperature control unit. Typically, the compressor is driven by a prime mover such as a diesel engine or an electric motor. The prime mover provides power to drive the compressor when the temperature control unit is operating. Most existing temperature control units are designed so that when maximum cooling capacity or xe2x80x9cpull-downxe2x80x9d is required to cool the conditioned space of the container, the compressor runs at a high speed. When the desired temperature or xe2x80x9cset-pointxe2x80x9d is achieved, the compressor speed is reduced. When pull-down is again required due to increased temperatures in the conditioned space, the compressor is again run at high speeds until the set-point is reached.
Running the compressor at high speeds during pull-downs creates various problems. First, the amount of power required to run the compressor at high speeds can be relatively high, thereby increasing the horsepower requirement of the compressor. With many of today""s compact temperature control units, the prime movers are simply too small to provide the power necessary to maximize the compressor""s capacity and pull-down potential at high operating speeds.
Second, running the compressor at high speeds increases the noise output of the temperature control unit. Noise levels would be greatly diminished if the amount of high-speed compressor run time could be reduced.
Therefore, it is desirable to utilize a compressor control system that automatically increases the compressor""s capacity while reducing the amount of high-speed compressor run time, and therefore the peak power requirements and the noise generated by the compressor. The method of compressor control embodying the invention increases the compressor""s capacity and pull-down potential by monitoring and altering the loading characteristics of the compressor in relationship to the power available from the prime mover.
More specifically, the present method controls the operating conditions of the compressor based on a comparison of power requirements for the compressor operating under different loading conditions. The system continually monitors the power requirements of the compressor at the actual operating mode and compares that actual power requirement to the maximum power available from the prime mover. Based on this comparison, the system determines whether the compressor is operating in the proper mode with the appropriate loading conditions, or whether the operating mode of the compressor should be switched to maximize capacity.
By constantly monitoring the various operating modes of the compressor to achieve maximum capacity based on the available power, the compressor can achieve faster temperature pull-downs, even when the conditioned space experiences regular door openings. Additionally, the amount of high-speed run time of the compressor is reduced, thereby reducing the noise level of the compressor and the temperature control unit as a whole.
More specifically, the invention provides a method of controlling a compressor in a transport temperature control unit. The temperature control unit includes a prime mover providing power to the compressor. The compressor has a power requirement that varies depending on loading conditions of the compressor. The method includes determining the maximum power available from the prime mover, determining the power requirement of the compressor, and adjusting the loading conditions of the compressor so that the power requirement of the compressor substantially equals the maximum power available from the prime mover.
In one aspect of the invention, the maximum power available is determined via unit testing. The power requirement of the compressor can be determined using the equation:       P    =                  p        s            ·                        V          d                ⁡                  (                      RPM            60                    )                    ·              (                                            (              PR              )                                      (                                                n                  -                  1                                n                            )                                -          1                )            ·              (                  VE          AE                )              ,
or can be determined using look-up tables. In another aspect of the invention, adjusting the loading conditions of the compressor includes changing at least one of the speed of the compressor and the suction pressure of the compressor.
The invention also includes a method of compressor control to maximize the pull-down capacity of a temperature control unit with respect to a predetermined maximum power available to run the compressor. The method includes starting the compressor at a low speed, and at suction pressure setting that is equal to or lower than a predetermined maximum value, varying the suction pressure as permitted by the maximum amount of power available to run the compressor until the suction pressure reaches a maximum suction pressure setting, and after the suction pressure reaches a maximum suction pressure setting, increasing the speed of the compressor as permitted by the maximum amount of power available to run the compressor. Once the predetermined maximum power available to run the compressor is surpassed, the method further includes reducing the compressor speed to a minimum speed setting before reducing the suction pressure.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.